Most small molecule drugs bind enzymes or receptors in tight and well-defined pockets. On the other hand, protein-protein interactions are notoriously difficult to target using small molecules due to their large contact surfaces and the shallow grooves or flat interfaces involved. E3 ubiquitin ligases (of which hundreds are know in humans) confer substrate specificity for ubiquitination, and therefore, are more attractive therapeutic targets than general proteasome inhibitors due to their specificity for certain protein substrates. The development of ligands of E3 ligases has proven challenging, in part due to the fact that they must disrupt protein-protein interactions. However, recent developments have provided specific ligands which bind to these ligases. For example, since the discovery of nutlins, the first small molecule E3 ligase inhibitors, additional compounds have been reported that target E3 ligases, but the field remains underdeveloped.
One E3 ligase with therapeutic potential is Cereblon, a protein that in humans is encoded by the CRBN gene. CRBN orthologs are highly conserved from plants to humans, which underscores its physiological importance. Cereblon forms an E3 ubiquitin ligase complex with damaged DNA binding protein 1 (DDB1), Cullin-4A (CUL4A), and regulator of cullins 1 (ROC 1). This complex ubiquitinates a number of other proteins. Through a mechanism which has not been completely elucidated, cereblon ubquitination of target proteins results in increased levels of fibroblast growth factor 8 (FGF8) and fibroblast growth factor 10 (FGF10). FGF8 in turn regulates a number of developmental processes, such as limb and auditory vesicle formation. The net result is that this ubiquitin ligase complex is important for limb outgrowth in embryos. In the absence of cereblon, DDB1 forms a complex with DDB2 that functions as a DNA damage-binding protein.
Thalidomide, which has been approved for the treatment of a number of immunological indications, has also been approved for the treatment of certain neoplastic diseases, including multiple myeloma. In addition to multiple myeloma, thalidomide and several of its analogs are also currently under investigation for use in treating a variety of other types of cancer. While the precise mechanism of thalidomide's anti-tumor activity is still emerging, it is known to inhibit angiogenesis. Recent literature discussing the biology of the imides includes Lu et al. Science 343, 305 (2014) and Krönke et al. Science 343, 301 (2014).
Significantly, thalidomide and its analogs e.g. pomolinamiode and lenalinomide, are known to bind cereblon. These agents bind to cereblon, altering the specificity of the complex to induce the ubiquitination and degradation of Ikaros (IKZFI) and Aiolos (IKZF3), transcription factors essential for multiple myeloma growth. Indeed, higher expression of cereblon has been linked to an increase in efficacy of imide drugs in the treatment of multiple myeloma.
Androgen Receptor (AR) belongs to a nuclear hormone receptor family that is activated by androgens, such as testosterone and dihydrotestosterone (Pharmacol. Rev. 2006, 58(4), 782-97; Vitam. Horm. 1999, 55:309-52.). In the absence of androgens, AR is bound by Heat Shock Protein 90 (Hsp90) in the cytosol. When an androgen binds AR, its conformation changes to release AR from Hsp90 and to expose the Nuclear Localization Signal (NLS). The latter enables AR to translocate into the nucleus where AR acts as a transcription factor to promote gene expression responsible for male sexual characteristics (Endocr. Rev. 1987, 8(1):1-28; Mol. Endocrinol. 2002, 16(10), 2181-7). AR deficiency leads to Androgen Insensitivity Syndrome, formerly termed testicular feminization.
While AR is responsible for development of male sexual characteristics, it is also a well-documented oncogene in certain forms cancers including prostate cancers (Endocr. Rev. 2004, 25(2), 276-308). A commonly measured target gene of AR activity is the secreted Prostate Specific Antigen (PSA) protein. The current treatment regimen for prostate cancer involves inhibiting the androgen-AR axis by two methods. The first approach relies on reduction of androgens, while the second strategy aims to inhibit AR function (Nat. Rev. Drug Discovery, 2013, 12, 823-824). Despite the development of effective targeted therapies, most patients develop resistance and the disease progresses. An alternative approach for the treatment of prostate cancer involves eliminating the AR protein. Because AR is a critical driver of tumorigenesis in many forms of prostate cancers, its elimination should lead to therapeutically beneficial response.
There exists an ongoing need in the art for effective treatments for diseases, especially cancer, prostate cancer, and Kennedy's Disease. However, non-specific effects, and the inability to target and modulate certain classes of proteins altogether, such as transcription factors, remain as obstacles to the development of effective anti-cancer agents. As such, small molecule therapeutic agents that leverage or potentiate cereblon's substrate specificity and, at the same time, are “tunable” such that a wide range of protein classes can be targeted and modulated with specificity would be very useful as a therapeutic.